| The main theme of this research work was investigation of the precursor-ceramic conversion process for some polycarbosilane polymers, (-RR{dollar}rmspprime SiCHsb2{dollar}-){dollar}rmsb{lcub}n{rcub},{dollar} known as the poly(silylenemethylene)s (PSMs), where R and R{dollar}spprime{dollar} are either hydrogen or bridging oxygen. The pyrolysis chemistry was characterized by elemental analysis, thermogravimetric analysis, liquid and solid state NMR spectroscopy, FTIR, and mass spectrometric analysis of the gaseous pyrolysis products. The strategy included three steps:; First, linear poly(silaethylene), (SiH{dollar}rmsb2CHsb2{dollar}), PSE, was synthesized by ROP and examined as potential precursor to silicon carbide. This was one of the limiting cases where in (-RR{dollar}rmspprime SiCHsb2{dollar}-){dollar}rmsb{lcub}n{rcub},{dollar} R=R{dollar}spprime{dollar}=H. The conversion process was studied by the examination of the gaseous species evolved during pyrolysis using a mass spectrometer. The results suggested that molecular H-transfer and elimination reactions involving silylene intermediates occurred initially and caused the crosslinking of the polymer between 300 and 420{dollar}spprime{dollar}C. Free radical reactions became operative and were the main mechanisms occurring above 420{dollar}spprime{dollar}C. The unusually high ceramic yield of linear PSE (ca. 80%) suggested that the SiH{dollar}rmsb{lcub}x{rcub}{dollar} groups in this polymer provided a latent reactivity that could be "turned on" by heating, thereby allowing the formation of a network structure that resists fragmentation.; Second, polycarbosilane/siloxane hybrid polymers, (Si(O)CH{dollar}rmsb2rbracksb{lcub}n{rcub},{dollar} were synthesized by sol-gel processing and were pyrolyzed to silicon oxycarbide ceramics. This was the other limiting case where in (-RR{dollar}rmspprime SiCHsb2{dollar}-){dollar}rmsb{lcub}n{rcub},{dollar} R=R{dollar}spprime{dollar}=bridging or terminal oxygens. The gels were converted into silicon oxycarbides that contain a statistical distribution of the five possible SiC{dollar}rmsb{lcub}4-x{rcub}Osb{lcub}x{rcub}{dollar} environments between 600 and 1000{dollar}spprime{dollar}C. This rearrangement of the Si environments was attributed to the redistribution reactions involving the exchange of Si-O and Si-C bonds during the latter stages of the pyrolysis, likely facilitated by the Si-OH-induced attack on the Si-CH{dollar}sb2{dollar}-Si linkages.; In addition, the microstructure of the gels and their pyrolytic products was investigated by N{dollar}sb2{dollar} adsorption-desorption test (the BET test). The results suggested that the investigated samples are microporous solids with relatively high surface areas even at 1000{dollar}spcirc{dollar}C, indicating the potential interest of these samples as microporous materials.; Finally, a mixture system was synthesized by introducing oxygen into the (SiH{dollar}rmsb2CHsb2{dollar}), polymer purposely to obtain a model (SiH{dollar}rmsb2CHsb2rbracksb{lcub}n{rcub}rbrack Si(O)CHsb2rbracksb{lcub}m{rcub}{dollar} system which has a variable and controllable amount of oxygen. This was the intermediate case between the two extremes. In this system, the pyrolysis mechanisms which worked in the two extreme cases also operated here along with a new mechanism resulting the formation of part of the total H{dollar}sb2{dollar} between Si-OH and Si-H groups. |
